I've noticed that quite a few MSN/WLM users have been duped into copying text with fancy (and ugly, in my opinion) signs and symbols into their personal message area, or What I'm listening to area. This effectively disables your Messenger account, and you won't be able to sign in.
Solution for XP users:
Go to C:\Documents and Settings\
your account name\Local Settings\Application Data\Microsoft\Windows Live Contacts\
the E-mail you used the bug on\real. Delete the weird numbered/lettered text. Reboot your computer and sign in.
For Vista users:
Click on Start > Run. In the box, type C:\Users\
your name\Local Settings\Application Data\Microsoft\Windows Live Contacts. Delete the folder with your email address. Reboot, and sign in.
The moral of the story (and I have to be blunt here): don't be stupid/naive/foolish to copy text just because someone asked you to 